Boris Johnson's decision to support the EU Brexit campaign is "a decision in search of a justification", shadow foreign secretary Hilary Benn has told ITV's Good Morning Britain.
Mr Benn, who will campaign to remain, said the Mayor of London's explanatory newspaper article had accepted there are "risks" to Britain leaving the EU.
David Cameron has warned that Britain has "100 days to secure our future", as a poll suggests the final result hangs in the balance.
The EU referendum is a choice between "the devil and the deep blue sea", one of the UK's most senior business leaders is to say today.
Britain's car industry is an economic success story, and with uncertainty over the impact of Brexit, it wants to stay put.